Voltage ratio discriminator

ABSTRACT

A circuit for providing an output which is the ratio of the peak input voltage divided by the average input voltage to the circuit. This circuit includes means to first provide for generating both the peak input and the rectified average or background input which provides an output which serves as a means to control the overall operation of the initial averaging means. There is further provided means to distinguish a peak value from the first mentioned average to provide the output specified.

United States Patent 11 1 Davis 7 1 1 VOLTAGE RATIO DISCRIMINATOR [75] lnventor: Richard K. Davis, Roanoke, Va.

[73] Assignee: General Electric Company, Salem,

[22] Filed: May 30, 1972 [21] Appl. No.: 257,917

[4 1 Dec. 25, 1973 1,869,331 7/1932 Ballantine 325/402 X 2,302,520 11/1942 Bingley 329/192 X 2,531,935 1,1/1950 Doelz 330/96 X 3,576,452 4/1971 Smith 330/59 X 3,609,407 9/1971 Garuts 307/235 A X Primary ExaminerA1fred L. Brody Att0meyArnold E. Renner et a1.

7 Claims, 6 Drawing Figures PEAK DETECTOR RECTIFlED [56] References Cited UNITED STATES PATENTS 3,145,345 8/1964 Squillaro et a1 330/96 X 2,627,022 1/1953 Anderson 330/96 X 2,724,089 11/1955 Ruston 329/179 X A G C ATIPLl Fl E R r AVERAGE CIRCUIT PATENTEDnuzzs ms SHEET 10F 4 PEAK DETECTOR IO I4- 22 A60 AMPLIFIER A A E M g I l2 8 RECTIFIED I AVERAGE CIRCLHT PRIOR ART 39' g 4-0 PEAK f V DETECTOR AGO RECTIF'ED AMPLI Fl E R QYQ$ FIG.2.

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VVK/ iNPUT OF OPERATIONAL AMPLIFIER I02 X VOLTS RECTIFIED AVERAGE DETECTOR O Vol-Ts OUTPUT VOLTAGE RATIO DISCRIMINATOR BACKGROUND OF THE INVENTION In certain applications it is desirable to provide a signal which is proportional to the ratio between a peak value which may randomly appear throughout a time period and a background signal value which tends to be of a more constant value. An example of such a use is in what is commonly known as signature analysis. In signature analysis there is normally provided a transducer of some form which senses some variation within the system being analyzed to provide an electrical output proportional to that variationJFor example, in the signature analysis of a bearing, a transducer which may be of a piezoelectric type is placed adjacent to the bearing, such as on the housing. By the proper interpretation of the electrical output of thepiezoelectric crystal, resulting from the vibrations emanating from the bearing in its operating mode, it may be determined that the bearing is failing for one or more reasons. This is possible because particular faults will exhibit certain characteristics. Thus, a ball bearing which is pitted or has a flattened side will, during its operation exhibit'certain physical vibrations which may be converted into electrical signals and these signals are in turn capable of interpretation to predict an impending failure of the bearing long before there is other-physical evidence of an impending failure or there is complete breakdown of the machine. Thus, periodic maintenance may beappropriately scheduled for that device (bearing) at a time which does not necessitate the removal of the machine from normal operations.

Because there is a certain amount of normal vibration and friction in the operation of any mechanical device such as a bearing, an absolute measure of the total vibration as it is reflected through electrical quantities is not sufficient to detect a fault. In the bearing example being used, it should be realized that because of varying operational circumstances the actual total vibration over a period of time will vary a considerable amount. Such factors as temperature, loading and speed will all affect the bearing vibrations, whether the bearing be normal or defective. It is, however, a normal situation that a fault or defect will exhibit a greater instantaneous vibration than the normal or what may be termed background vibration of the operation of the mechanical device, in this case a bearing. Thus the need to be able to accurately detect a peak value of such a vibration as opposed to what has been termed background, even though each may vary in magnitude, becomes essential.

It is known in the prior art, as will be described in greater detail later in this specification, that a suitable transducer may be used to provide an electrical signal which is representative of vibrations-within the device being considered. This signal is delivered to a suitable amplifier often in the form of an automatic gain control (AGC) amplifier having a feedback from its output which serves as a control signal. The output from the AGC amplifier serves as an input to two identifiable circuit elements one of which provides a rectified average value of the output of the AGC amplifier and the second of which provides a signal proportional to the peak value of amplifier. The outputs from these two elements are then provided as inputs to a divider circuit which serves to provide an output which is the quotient of the peak voltage divided by the average voltage.

While circuits of this nature are known, they suffer from at least one of two common faults both of which center around the divider circuit. In order to provide a circuit whichgives an accurate indication, the divider circuit is relatively very expensive and must be built with a great deal of precision. If it is not so designed and built it is inaccurate and the readings provided are not of the degree of accuracy desirable in many applications.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a voltage ratio'discriminator which is both accurate and relatively inexpensive.

It is a further object to provide a voltage ratio discriminator which is comprised of a fewer number of parts or circuit elements than those heretofore known.

The voltage ratio discriminator of the present invention-provides an inexpensive and accurate output voltage without the use of the divider circuit prevalent in the prior art. To achieve this action the output from an AGC amplifier is fed to a proportioning circuit the output of which forms a feedback signal to the AGC amplifier to thus hold the output of that amplifier at a prescribed average level. Thus, the output of the AGC amplifier which is then fed to a peak detector has a peak component equal to the ratio of peak to average of the input signal and the voltage divider circuit has been eliminated.

DESCRIPTION OF THE DRAWINGS The foregoing and other objects of the present invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, reference is made to the accompanying drawings in which:

FIG. I is a block diagram of a voltage ratio discriminator inaccordance with that known in the prior art.

FIG. 2 is a block diagram of the voltage ratio discriminator in accordance with the present invention.

FIGS. 3 and 4 are detailed schematic diagrams showing suitable circuits for use in the system diagram shown in block form in FIG. 2.

FIGS. 5 and 6 are voltage waveforms at various points within the circuit of the present invention which are useful in the understanding of the present invention.

DETAILED DESCRIPTION Referring first to FIG. 1 wherein there is shown in block form what has been previously described as the prior art, it is seen that the discriminator includes an automatic gain control (AGC) amplifier 10 to which there is applied as an input a signal which is the composite of a first or background signal and a second signal at random occurrence and having a peak amplitude which may be several times greater than that of the background. In the case of the present example of bearing signature analysis, the latter signal could result from a pit on the bearing surface. In the case of signature analysis, as is being illustrated in FIG. 1, the input to the amplifier 10 is provided by a suitable transducer capable of converting mechanical energy into electrical energy such as a piezoelectric crystal 12. The output from the crystal I2 is supplied to a preamplifier 14 to provide a signal of a level sufficient to drive amplifier 10. As an example of the type of input which might be experienced by the amplifier 10 reference is made to the upper wave form shown in FIG. 5 labeled transducer output. In that depiction there is shown a background signal of varying magnitude with randomly spaced spikes of higher level. The AGC amplifier serves the purpose of providing an output which is similar to its input excepting that magnitude of the two components of the input signal are maintained within predetermined bounds. The predetermined bounds are maintained, in the embodiment shown in FIG. 1, by a feedback signal from the output of the AGC amplifier to that amplifier shown in the figure as a path 16.

The output of the AGC amplifier 10 of FIG. 1 forms an input to a rectified average circuit 18 which performs, in essence, full wave rectification and filtering to the output of the amplifier l0 and provides, at its output, a signal which has a voltage level proportional to the rectified average of the input signal. The output of the AGC amplifier 10 also serves as an input to a peak detector circuit 20 which provides an output signal having a voltage level which is proportional to the value of the peak of its input signal. The output of the rectified average circuit 18 and the peak detector 20 are then supplied to a standard divider circuit 22 which provides an output which approximates the ratio of the voltage of the peak detector 20 divided by the voltage level from the rectified average circuit 18. In FIG. 1 this is illustrated as a signal Vp/Vavg. appearing on line 24 of that Figure.

Referencing now FIG. 2, which illustrates the present invention in block form, it is seen that while the circuit employs most of the same elements as were present with the prior art system shown in FIG. I there are two major distinctions. The first of these distinctions which is readily apparent is that the divider circuit 22 of FIG. 1 has been eliminated and that the output which is the ratio of peak voltage to average voltage is derived directly from the peak detector. The second immediately obvious distinction is that the feedback signal to the AGC amplifier is'from a rectified averaging circuit. Referencing nowFIG. 2 in detail and once again employing, for purposes of explanation the application of the circuit of the present invention to signature analysis, it is seen that there is provided a suitable signal generating means such as a transducer 30 which, as before, may be of the piezoelectric variety and which provides an electrical output signal which is a function of mechanical stresses applied to the transducer all in a manner well known in the art. As was the case in FIG. 1, and once again using the pitted bearing example, the output of the transducer may have the wave shape shown in the uppermost wave form of FIG. 5. That is, the total signal includes a first portion of relatively low level which may be again called background a second signal portion of a much larger amplitude which is superimposed thereon and which would indicate a fault in the device being considered.

The output from the transducer 30 inputs to a suitable preamplifier 32 which serves to amplify the signal from the transducer 30 to a level suitable for use by an AGC amplifier 34. In the present embodiment amplifier 32 is preferably a charge amplifier although this is a matter of design preference and a voltage amplifier could be used; it being necessary only that the amplifier 32 be capable of providing suitable amplification of the signal resulting from the source of transducer 30. The output of the amplifier 32 is a voltage signal whose amplitude or magnitude is proportional to the output of the transducer 30.

The AGC amplifier 34 is an amplifier whose gain is controlled by a control signal which in the present embodiment is supplied from a proportioning circuit shown as a rectified average circuit 36 by way of a feedback path 38. In the present invention because the control signal is derived from the circuit 36, the output of the AGC amplifier has a waveform which has had the two component parts of its input waveforms averaged such that there is provided a substantially constant level background signal as well as substantial level peak signals. This waveform is illustrated in the second waveform of FIG. 5 labeled AGC amplifier output".

The output from the AGC amplifier forms an input to the rectified average circuit 36 which serves the function of providing full wave rectification and averaging to provide at its output a voltage signal which is of a level proportional to the average of the rectified signal from the AGC amplifier 34. By this means the AGC amplifier output may be held constant regardless of the amplitude of the input. Thus, the voltage appearing at the output of the AGC amplifier 34 is a constant value excepting that it is also provided with certain peaks which are present within the input signal. The output of the AGC amplifier 34 is also the input to a peak detector 39 and, inasmuch as the average value of the AGC amplifier has been held constant the peak detector sensing the peaks will provide an output the voltage of which is proportional to the ratio of the peak to the average value. That is, by properly scaling the output of the rectifier 36, the average value of the background is known and hence by detecting peaks there will be provided an output from the peak detector 39 which is the function directly of the ratio of peak voltage to average voltage.

FIGS. 3 and 4 show in detail one form the various components of the present invention illustrated in block form in FIG. 2. In FIG. 3 there is again shown a source of input signals such as a piezoelectric crystal or accelerometer 30 which supplied an input to the charge amplifier 32. This input is applied to an inverting input of a standard operational amplifier 50 which is provided with a capacitor feedback path through a capacitor 52. The other or normal input to the amplifier 50 is from a common closed loop bus 54. The output level of the amplifier 32 is a function of the gain of the operational amplifier 50 which is determined by the value of the capacitor 52 and will be a prescribed number of volts per coulomb. (It should be explained at this point that throughout the description of FIGS. 3 and 4 there are shown several operational amplifiers and that as a standard practice each of these amplifiers would be provided with suitable voltage supply; for example, from buses of positive and negative 12 volts all in a manner well known in the art. These power connections to these several operational amplifiers have, however, been omitted for purposes of clarity).

The AGC amplifier 34 is capacitively coupled to the amplifier, 32 by way of a coupling capacitor 55 which connects to the junction of a voltage divider network comprised of two resistors 64 and 66 extending between a positive bus 53 and a common bus 54. Included within the AGC amplifier 34 is a standard operational amplifier 56 whose gain is determined by the ratio of two resistors 58 and 60 connected in series between the common bus 54 an the output of the amplifier 56 with their junction point forming an input to the inverting input of the operational amplifier 56. In order to achieve the substantially constant voltage output from the AGC amplifier 34 it is necessary that this amplifier have variable gain capability. This variable gaincapability is achieved through the use of a field effect transistor 59 which has its source electrode connected as an input to the normal input of the operational amplifier 56. In essence, the field effect transistor 59 appears to the normal input of the operational amplifier56 as a variable resistance element with the magnitude of that resistance being determined by the voltage on the gate electrode of the field effect transistor 59. As was previously indicated, the input to the AGC amplifier 34 from the amplifier 32 is by way of a coupling capacitor 55 which is connected at the junction of the -two'resistors 66 and 64, resistor 66 being of the potentiometer'type and having a slider contact 67. Slider 67 of potentiometer 66 is capacitively coupled to the drain electrode of field effect transistor {59 by way of a coupling capacitor 62 and the purpose 'of the variable input by way of slider contact 67 is to maintain the input signal to the operational amplifier within the general level for the presecribed application. Field effect transistor 59, as previously stated, appears to the operational amplifier 56 as a variable resistance and this resistance, appearing between the drain and source electrodes is a function of the voltage appearing on its gate electrode. The variable resistance of the field effect transistor 59 appears in series with a resistor 68 the free end of which is connected to the common bus. The transistor 59 and the resistor 68 form a voltage divider with the midpoint of the divider connected to the normal input to the operational amplifier 56.

The resistive value of the field effect transistor 59, as was previously stated, is a function of the voltage on its gate electrode and it is this resistance which determines the overall gain of the AGC amplifier 34. To vary the voltage on the gate electrode there is provided on line 38 (see FIG. 2) a signal from the rectified average circuit 36 which is applied to the series combination of a variable resistor 72, a resistor 74 and a resistor 76 the free end of which is tied to the negative bus. The junction of the resistors 74 and 76 forms the input to the base of a transistor 70 which has its emitter connected to the common bus and its collector connected to the junction of the gate electrode of the field effect transistor 59 and a resistor 82 the free end of which is connected to the positive bus.

The manner in which the output from the rectified average current circuit 36 affects the output of the AGC amplifier is as follows. The signal from the rectified average circuit 36 appearing on line 38 and as seen by the base of the transistor 70 through the voltage divider network 72, 74 and 76 will affect the degree of conduction of the transistor 70. That is, if that signal is of sufficiently high value as seen by the base of the transistor 70 to render the transistor 70 conductive, conduction of transistor 70 will lower the voltage on the gate electrode of the field effect transistor thus increasing its effective resistance and thus lowering the over gain of the amplifier 34. Amplifier 34 will, in turn, provide a lower value signal to the rectified average current circuit 36. If on the other hand the signal on line 38 is of a lower value, the transistor 70 will tend to conductto a lesser degree to the point of being turned off. This will, in turn, raise the voltage of the gate electrode of the field effect transistor 59 lowering its effective resistance and thereby raising the output from the operational amplifier 56 and hence the output from the AGC amplifier 34. The signal thus applied to the rectified output circuit 36 tends to raise its output signal for application, once again via line 38, to the amplifier circuit 34. Resistor 72 is preferably made variable in order to adjust the magnitude of the output of the AGC amplifier 34 to a known calibrated value.

Completing the depiction shown for the AGC amplifier 34 in FIG. 3 is a capacitor 78 and a resistor 80 connected in series between the collector and base of transistor 70 which series network serves as a stabilizing RC network to provide overall stability within the AGC circuit 34. The amount of the gain of the amplifier is determined by the degree of conduction of the transistor 70. If transistor 70 is fully on then the gate electrode of field effect transistor will be near zero volts and when transistor 70 is nonconducting, the gate will be at a relatively high value due to the effect of the resistor 82 which connects the gate to the positive bus as previously described. Connected between the positive bus and the common bus is a variable resistor 84 having its slider contact forming an input to a resistor 86 which is connected to the inverting input of the operational amplifier 56. The resistor network 84 and 86 provides a means for adjusting the offset of the amplifier 56 to produce a desired DC output voltage. A capacitor 88 connected in the feedback loop between the output of the amplifier 56 and the inverting input of that amplifier serves as a filter capacitor to eliminate spurious noise signals from being applied to the input of that amplifier. The time constant of the capacitor 88 and the resistor 60 is such that the frequencies of interest will be preserved'at the output of the operational amplifier while spurious noise will be rejected.

Referencing now FIG. 4 in conjunction with FIG. 2 it is seen that the output from the AGC amplifier 34 forms input to both the rectified average circuit 36 and the peak detector circuit 39. Reference will first be made to the rectified average detector circuit 36, which will be explained with respect to the waveforms shown in FIG. 6. While the FIG. 6 waveforms are of the idealized nature (starting with a sine wave) and while it is recognized that this idealized situation will not exist in practice, the analysis and the explanation are nevertheless valid.

Referencing now FIG. 4 it is seen that the rectified average circuit 36 includes two operational amplifiers and 102. Amplifier 100 along with its associated circuitry performs a full wave rectification function while amplifier 102 serves as a precision amplifier and filtering network. The signal from theAGC amplifier 34 is applied through a resistor 104 to the inverting input of operational amplifier 100, the normal input of which is connected to the common bus 54. This input is shown in FIG. 6 in the idealized condition as a sine wave (uppermost waveform). The output of the amplifier 100 is connected to the anode of a clamping diode 106 the cathode of which is tied to the inverting input of that amplifier. The output of the amplifier 100 is also connected to the cathode to a second diode 108 the anode of which is connected to the junction of a resistor 110 and a second resistor 112 the free end of the resistor l 12 also being connected to the inverting input of the amplifier 100. The gain of the amplifier 100 is determined by the ratio of the resistors 104 and 112 when the amplifier 100 is in its linear region; that is, when the input signal is positive. When the input signal is negative the output of the amplifier 100 is clamped by diode 106 to a maximum of approximately six-tenths volt positive. Diode 108 serves to compensate for the voltage drop across diode 106 in a reverse direction; that is, it will also have a drop of approximately sixtenths of a volt such that the voltage, when the input signal to the operational amplifier 100 is negative, appearing at the anode of diode 108 will be approximately zero volts.

In the preferred embodiment of the invention, resistors 104 and 112 are selected such that the gain of the amplifier 100 is approximately unity. Inasmuch as the negative half of the input wave is suppressed the output of the amplifier 100 as it is seen by the current through the resistor 110 is a plurality of negative going half cycles of a sine wave which are in phase with the positive going half cycles of the input wave and are of equal magnitude. The output of amplifier 100 is shown in the second graph of FIG. 6 identified as i The output signal from the AGC amplifier 34 is also supplied by way of two resistors 114 and 116 to a summing junction 118 to which is also connected the free end of the resistor 110. If resistor 114, 116 and 110 are all the same value and it is assumed that there is no signal attenuation in the circuitry directly associated with the amplifier 100, the signals applied to the junction 118 will be in the ratio of 2: l. The first of these signals will be the negative half sine wave pulses from the amplifier 100 as seen through resistor 110 and the second of these will be the full sine wave input as seen through resistors 114 and 116 with this latter signal having a peak magnitude of approximately one-half of the peak value of that from the amplifier 100. At junction 118, therefore, the sum of these two signals is a signal which is a negative full wave rectification of the input signal which has a value of about one-half of that of the input. This is illustrated in FIG. 6 by the third waveform designated as Input of operational amplifier 102. This composite is applied to the inverting input of amplifier 102 which amplifier has its normal input connected to the common bus and the gain of which is determined by the value of a resistor 120 connected in a feedback path between the output of the amplifier 102 and the junction 118. The output voltage of the amplifier 102 is a voltage which is proportional to the summation of the input currents i and i (scaled by the feedback through resistor 120) and is essentially a constant DC level. (Depicted in FIG. 6 as X volts on the lower most graph) A capacitor 122 is connected between the output of the amplifier 102 and the junction 118 serves as a filtering capacitor. The output from the circuit 36 appears on line 38 and is supplied as the control input to the AGC amplifier 34 as was described with respect to FIGS. 2 and 3.

The output from the AGC amplifier also forms an input to the peak detector 39. When the output of the AGC amplifier 34 goes positive, the signal is applied through a diode 130 which is polled in a direction to pass positive going signals. The value of a peak signal serves to charge a capacitor 132, connected between the cathode of diode 130 and the common bus 34, positive to its upper plate as shown in FIG. 4. The value of the charge on capacitor 132 corresponds to the peak value of the applied signal. When the input signal falls below this peak value, the positive charge of capacitor 132 will reverse bias diode rendering it nonconductive and thus maintain the charge on the capacitor. In parallel with the capacitor 132 is a resistor 134 which is of high ohmic value and which provides a bleeder function for the capacitor 132 such that the capacitor 132 may discharge very slowly between peak signal inputs. The upper plate of capacitor 132 is connected to the normal input of an operational amplifier 136 which is simply a follower type amplifier which provides at its output a signal or voltage which is proportional to the input voltage and provides impedance transformation to give a low impedance output. Connected between the output of the amplifier 136 and its inverting input is a parallel combination of a diode 138 and a resistor 140 which form a feedback path to make the amplifier 136 of the inverting follower type. As is common, when the noninverting input of amplifier 136 goes positive the output follows it and the diode 138 pulls the inverting input positive. Since the input to the peak detector 39 is a signal whose average value is a constant, as defined by the AGC amplifier 34, the output of the detector is proportional the peak value of tha signal. Thus, for example, if the input has as an average of 1 volt then the output would constitute the ratio between the peak value and the average input. It is, of course, to be understood that the I volt value is simply illustrative and that the average value could be of any prescribed magnitude with the output of the circuit being proportionally scaled. Thus it is seen that there is provided a relatively simple and economical circuit which provides at its output a signal which is proportional to the ratio between the peak voltage input and the averag voltage input.

While there has been shown and described what is at present considered to be the preferred embodiment to the invention, modifications thereto will readily occur to those skilled in the art. For example, as was earlier mentioned in this description, the reference to which the peak is to be measured could be of some value other than average, for example, the RMS value. In such a situation the rectified average circuit shown and described would be replaced by a circuit which provides an RMS value as opposed to an average value. Such circuits are known to those skilled in the art but are somewhat more complex than the averaging circuit shown. However, all other apsects of the circuit of the invention would be the same and the same fundamental principles would be applicable. It is, therefore, desired that the claims concluding this specification not be limited to the specific embodiment shown, but that they be interpreted in accordance with the true spirit and scope of the invention.

What is claimed is:

1. A voltage ratio discriminator comprising:

an amplifier adapted to receive an input signal of a composite nature having a background portion of a first general level and a peak portion of a second general level higher than said first, said amplifier further including control means to vary the rate of amplification thereof in accordance with the value of a control signal applied to said control means; proportioning circuit means coupled to said amplifier for receiving the output signal from said amplifier and having an output coupled to said control means of said amplifier, said proportioning circuit means producing a control signal bearing a prescribed relationship with respect to the output signal received from said amplifier; and

peak detecting means coupled to the output of said amplifier and responsive to that portion of the output signal received from said amplifier which represents said peak portion of said input signal to produce a final signal representative of the difference in magnitude between said background portion and said peak portion, said final signal representing the ratio between the amplitude of said peak signal and said background portion.

2. The invention in accordance with claim 1 wherein said proportioning circuit provides a control signal which is the average value of the output from said amplifier.

3. The invention in accordance with claim 1 wherein said proportioning circuit provides a control signal which is the RMS value of the output from said amplifier.

4. A signature analysis circuit comprising:

a. a transducer for providing a signal representative of existing properties with respect to a device being analyzed, said signal comprised of a first portion representing a background condition and a second portion representing a fault condition of the device;

b. an amplifier for receiving the signal from said transducer and for providing a composite output signal with a substantially constant amplitude portion upon which is randomly superimposed a peak portion of substantially constant amplitude all in accordance with the value of a control signal applied to a control means within said amplifier;

c. a proportioning circuit for receiving said composite output signal and for providing said control signal, said control signal bearing a prescribed relationship with respect to said composite output signal; and,

d. a peak detecting means responsive to the peak portion of the composite output signal of said amplifier to produce a final signal having a value in fixed proportion to the value of said control signal.

5. The invention in accordance with claim 4 wherein said transducer is a piezoelectric crystal.

6. The invention in accordance with claim 4 wherein said proportioning circuit provides a control signal which is the average value of the composite output signal.

7. The invention in accordance with claim 4 wherein said proportioning circuit provides a control signal which is the RMS value of the composite output signal. =l= 

1. A voltage ratio discriminator comprising: an amplifier adapted to receive an input signal of a composite nature having a background portion of a first general level and a peak portion of a second general level higher than said first, said amplifier further including control means to vary the rate of amplification thereof in accordance with the value of a control signal applied to said control means; proportioning circuit means coupled to said amplifier for receiving the output signal from said amplifier and having an output coupled to said control means of said amplifier, said proportioning circuit means producing a control signal bearing a prescribed relationship with respect to the output signal received from said amplifier; and peak detecting means coupled to the output of said amplifier and responsive to that portion of the output signal received from said amplifier which represents said peak portion of said input signal to produce a final signal representative of the difference in magnitude between said background portion and said peak portion, said final signal representing the ratio between the amplitude of said peak signal and said background portion.
 2. The invention in accordance with claim 1 wherein said proportioning circuit provides a control signal which is the average value of the output from said amplifier.
 3. The invention in accordance with claim 1 wherein said proportioning circuit provides a control signal which is the RMS value of the output from said amplifier.
 4. A signature analysis circuit comprising: a. a transducer for providing a signal representative of existing properties with respect to a device being analyzed, said signal comprised of a first portion representing a background condition and a second portion representing a fault condition of the device; b. an amplifier for receiving the signal from said transducer and for providing a composite output signal with a substantially constant amplitude portion upon which is randomly superimposed a peak portion of substantially constant amplitude all in accordance with the value of a control signal applied to a control means within said amplifier; c. a proportioning circuit for receiving said composite output signal and for providing said control signal, said control signal bearing a prescribed relationship with respect to said composite output signal; and, d. a peak detecting means responsive to the peak portion of the composite output signal of said amplifier to produce a final signal having a value in fixed proportion to the value of said control signal.
 5. The invention in accordance with claim 4 wherein said transducer is a piezoelectric crystal.
 6. The invention in accordance with claim 4 wherein said proportioning circuit provides a control signal which is the average value of the composite output signal.
 7. The invention in accordance with claim 4 wherein said pRoportioning circuit provides a control signal which is the RMS value of the composite output signal. 